The role of nonlinear materials in high-speed applications such as optical switching, amplification, limiting and frequency conversion has created a need for an efficient method of characterizing nonlinear parameters. Many of these parameters can be characterized by the analysis of the index of refraction of a material. In particular, semiconductor materials exhibit a broad range of nonlinear effects with response times that span several orders of magnitude, owing to electronic nonlinearities, free-carrier effects, and thermal nonlinearities. Other materials may also exhibit properties which change over time, e.g., due to optical interaction or to environmental factors, and which also change the materials' index of refraction.
The presence of two or more nonlinear mechanisms can complicate the interpretation of optical nonlinearities because many techniques cannot distinguish between them. Quantitative information concerning the nonlinear index of refraction for optical materials is essential for the development of all-optical devices, such as opto-optical switches. Several techniques have been proposed for conducting this measurement, most of which are based on a direct interferometric measurement that uses a pump and probe technique.
One technique is to analyze temporal interference fringes to obtain the nonlinear index of refraction, as described in "Nonlinear-Index-Of-Refraction Measurement In A Resonant Region By The Use Of A Fiber Mach-Zehnder Interferometer", Applied Optics, Vol. 35, No. 9, Mar. 20, 1996, pages 1485-88. This technique uses fiber light guides in both the reference and measurement arms. Also included in each arm is an adjustable delay unit (AD) based on an optical fiber pigtailed graded index rod-lens pair, to vary the optical length of each arm.
The inventors have found that this technique is difficult to use to do measurements of bulk sample properties because of the difficulty in preparing an interface between the light guides in the measurement path and the sample to be measured. Often, it is possible that installing connecting light guides to the sample will result in some shift of its electrical properties. In addition, some samples cannot be connected directly to optical light guides.
Furthermore, according to this technique the pump pulses propagate in the optical fibers comprised in the interferometer arms; the inventors have observed that this sets a limit to the maximum pump power available for the measurements.
Another technique is disclosed in "Time-Resolved Absolute Interferometric Measurement Of Third-Order Nonlinear-Optical Susceptibilities", Journal of the Optical Society of America B, Vol. 11, No. 6, June 1994, pages 995-999. This technique, as illustrated in FIG. 1 of the paper, uses free space propagation of optical signals to measure nonlinear optical properties of bulk materials. A Mach-Zehnder interferometer compares the two beams (probe and reference) in amplitude and phase. The sample is located in the probe arm and interacts with the stronger collinear pump beam. The time delay .tau. between the pump and probe pulses provides the basis for a sampling interferometry.
The inventors have observed that the above techniques has disadvantages linked with using an optical measurement system wherein the light propagates completely in free space; in particular it is bulky and it needs careful alignment of all the optical components, what renders this technique difficult to use.
Other discussions of measurement of nonlinear properties can be found in "Femtosecond Time-Resolved Interferometry For The Determination Of Complex Nonlinear Susceptibility", Optics Letters, Vol. 16, No. 21, Nov. 1, 1991, pages 1683-1685 and "Interferometric Measurement Of The Nonlinear Index Of Refraction n.sub.2 Of CdS.sub.x Se.sub.1-x -Doped Glasses", Applied Physics Letters, Vol. 48, No. 18, May 5, 1986, pages 1184-1186.
U.S. Pat. No. 5,268,739 discloses a laser apparatus for measuring the velocity of a fluid. In the system disclosed, a laser beam is fed into a pipe through which a fluid is flowing. Particles in the fluid interfere with the light. The velocity of the fluid is calculated from this interference.